The present invention relates to a nozzle configuration for forming, charging and deflecting ink drops in an ink-jet printer, comprising a drop-formation nozzle, a charging electrode provided with a passage, and deflection plates, the central axes of said parts lying in line with one another.
Such a nozzle configuration is used in an ink-jet printer in order to charge drops which are formed under pressure, starting from a drop formation nozzle, while passing through the passage of a charging electrode, and subsequently to deflect them, through selective excitation of a set of deflection plates, depending on the final destination of a particular drop or series of drops. The nozzle configurations can also be designed in such a way that the drops are charged selectively through selective excitation of the charging electrode; the deflection plates are then connected to a constant deflection voltage.
Such a known nozzle configuration shows problems in use which are largely due to blockages connected with pollution. Dried-up ink and dust collecting in the nozzle configuration can cause such problems, so that fault-free working of the nozzle configuration is not achieved.
Attempts have been made through a suitable ink formulation to prevent blockages caused by solid material elements from the ink. In that case blockages can still occur through dust particles from the environment caking on the nozzle, the charging electrode and the deflection plates. The risk of pollution as a result of ink drops settling on the nozzle configuration occurs mainly during starting up and ending of operation of the ink-jet printer.
Attempts have been made to provide solutions of a constructive nature to the problems described above. For example, an attempt was made to build into the pressure circuit of the drop-formation nozzle a quick-acting valve which opens only at relatively high pressure. The pressure in the nozzle is then built up in a relatively short time, which means that the time during which fluid comes out of the nozzle at low speed is reduced. The high-pressure pulse required for opening the quick-acting valve does, however, mean that there is a risk of damage to the nozzle and the valve itself, which is undesirable. The valve is also expensive, which adversely affects the price of the printer. In order to prevent pollution of the nozzle configuration, the passage in the charging electrode, which can be in the form of a bore or a slit, is often made much larger than is actually necessary to allow the jet through.
Such a large dimension of the passage in the charging electrode is, however, a disadvantage because in that case extraneous electric fields can affect the charge, resulting in an undesirable deviation in the charging of the drops. In order to counter this effect, the length of the charging electrode, measured in the direction of the path of the drops, must increase. Through such measures, there is, however, an increased risk of charged and uncharged drops merging, which can have an effect on the final sharpness of the image to be printed.
In view of the last-mentioned considerations, a relatively small passage in the charging electrode and a small dimension measured in the drop path direction would be precisely what is required.
Also, on account of the required overall space for arranging several nozzle configurations, it is very important to keep the dimensions of the various parts of the nozzle configuration as small as possible, and to minimize the intervals between them.
In that case the chance of pollution is, however, increased again.
The object of the present invention is to provide a solution to the above-mentioned pollution problems, while at the same time providing a nozzle configuration with very small dimensions and distances between parts, which permits placing in an arrangement of great density.